This invention relates to a camera employing an automatic focusing unit for detecting an optical image through a photographic lens and for outputting an image signal, and more particularly to an arrangement for mounting the automatic focusing unit on the camera.
Various cameras provided with automatic focusing systems have recently been developed and brought into commercial markets.
In single-lens reflex cameras, an AF (automatic focusing) -sensor module operating as an automatic focusing unit for detecting the condition of optical image formed on the focal plane using an image sensor is generally employed. The module is disposed in the camera in such a fashion that its light receiving surface is placed at the optically equivalent position with a film plane.
Since the precision in the positional arrangement of the AF-sensor module in the camera directly affects the focusing performance, its light receiving surface and the film plane have to be precisely equivalently positioned.
FIGS. 1 and 2 show an example of a conventional mounting arrangement of the AF-sensor module.
An AF-sensor module 2A is fitted to the undersurface of a bottom plate 12 of a mirror-box 1A accommodating a quick return mirror 10.
A square hole 12a of predetermined size is formed in the bottom plate 12 of the mirror-box 1A. Light C carrying an optical image from a photographic lens, not shown, and through the half-mirror portion formed at the center area of the quick return mirror 10 is reflected downwardly by a mirror 11 disposed at a predetermined angle on the rear side of the quick return mirror. The light C is then emitted from the square hole 12a out of the mirror-box 1A (FIG. 2).
Three mounting bosses 13 project from the undersurface of the bottom plate 12 and are arranged around the square hole 12a. Fitting plates 21 are provided on the case 20 of the AF-sensor module 2A, on which three bores 3a are formed in positional relations with the bosses 13, respectively. The AF-sensor module 2A is held at the bottom plate 12 of the mirror-box 1A by means of three mounting screws 3, each of which passes through the bore 3a and screwed into the mounting boss 13, respectively.
A spring 4 is put on the outer periphery of each mounting boss 13 so that the resilient force of the spring 4, held between the fitting plates 21 and the bottom plate 12 of the mirror box 1A, biases the AF-sensor module 2A in the direction away from the mirror box 1A. In other words, the resilient force of the springs 4 keeps biasing the fitting plate 21 of the AF-sensor module 2A toward the head-side of the mounting screws 3 so as to prevent an unsteady sway.
The optical distance between the undersurface of the bottom plate 12 and the light receiving surface 25a of the AF-sensor module 2A can be changed by fastening or loosening the mounting screws 3 while ensuring that the optical axis crosses the light receiving surface of the AF-sensor module 2A at a right angle, whereby the light receiving surface 25a of the AF-sensor module 2A can be optically equivalently positioned with the film plane.
The light C carrying the optical image emitted from the square hole 12a out of the mirror-box 1A is directed into the AF-sensor module 2A by way of a window 20a where a lens 22 is fitted, a mirror 23, and a lens 24, and then reaches the light receiving surface 25a of a CCD (Charge Coupled Device) sensor 25 of the AF-sensor module 2A to form an optical image. Then the optical image is converted into image signals by the CCD sensor 25. That is, the AF-sensor module 2A outputs the image signal. Based on the image signals outputted from the AF-sensor module 2A, a focusing data signal is obtained.
In the aforementioned arrangement for adjusting the position and angle of the AF-sensor module 2A by tightening or loosening the mounting screws 3 while maintaining the condition that the AF-sensor module 2A is forced against the heads of the three mounting screws by means of the respective springs 4, the use of many small precision parts (the springs 4 in particular) unavoidably results in high manufacturing cost with troublesome assembly work because a number of small parts must be assembled.
When the screwed amounts of the three mounting screws 3 differ substantially from each other, the AF-sensor module 2A tends to tilt accordingly. Even when the AF-sensor module 2A is simply assembled at the preceding stage of the adjusting process, all the three mounting screws 3 have to be uniformly tightened and this further results in the troublesome assembly work.
Further, since ambient light may invade the optical path, the process for adjusting the mounting position of the AF-sensor module on the mirror-box has conventionally been done in a dark-box, which also makes the assembly work troublesome.
Furthermore, when the assembled mirror-box and AF-sensor module are installed in a camera, because of an adjusting space between the undersurface of the bottom plate 12 and the surface of the AF-sensor module 2A, ambient light which causes the low performance of the AF-sensor module may invade the optical path of the image carrying light, and dust is allowed to penetrate through the adjusting space and consequently stick to the light incident portion thereof (on the surface of the lens 20a), which causing the low measurement accuracy. The disadvantage also is caused when dust is allowed to penetrate via the square hole 12a into the mirror-box 1A.